PpiMHCIIb2 expression outside the tentacle root. (A) PpiMHCIIb2 expression in the apical organ. White arrowheads point to paired structures localised near the balancers (b) and black arrowheads point to paired structures located near the epithelial papillae (ep). (A’) Summary of the PpiMHCIIb2 expression pattern in the apical organ. (B) PpiMHCIIb2 expression in the anal pores (Ap). (C) PpiMHCIIb2 expression in the female gonad. (C’) Higher magnification of the region boxed in (C), showing an oocyte (Oo) surrounded by its nurse cells (nc), both expressing PpiMHCIIb2. (D) Expression of PpiMHCIIb2 in a paragastric canal. (E) Expression of PpiMHCIIb2 along a comb row and in the neighbouring epithelium. White arrowheads indicate the tentacular plane, where a line of PpiMHCIIb2 expressing cells is observed. (E’) Higher magnification of the region boxed in (E). The black arrowhead points to the symmetrical expression spots in the middle of the comb. The star points to the “subterminal spot” of staining near the extremity of the comb. ao: apical organ; Ap: Anal pore; b: balancer cells; Cpf: Central zone of polar field; ep: epithelial papillae Mpf: Marginal zones of polar field; nc: nurse cell; Oo: Oocyte. Scale bars: A: 20 μm; B, D, E’: 50 μm; C, E: 100 μm; C’, D’: 10 μm.